Propellers driven by reciprocating (piston) engines are widely used in general aviation to economically propel aircraft, particularly those flown by individual private pilots. Costs for economical 2-4 passenger aircraft may begin at $100-300,000. Two or more radial blades shaped as airfoils may rotate about a hub axis, with the chord of each blade twisted out of the plane of rotation by a blade twist angle in order to set a positive angle of attack (AOA) with respect to the relative airflow. The angle of the relative airflow may be approximately calculated by vector summing the axial velocity (arising from aircraft motion) and the blade velocity at any given point along the span of the blade. In order to maintain a positive AOA that is less than a stall angle, the blade twist angle may need to become progressively smaller in moving from the inboard region of the blade to the outboard region because blade velocity is greater at the tip. Unfortunately, high drag may occur as the relative airflow at the tip approaches Mach 1, reducing efficiency, and may limit prop-driven aircraft to low subsonic speeds of less than approximately 350 miles per hour (mph), or approximately Mach 0.5. Prop-driven aircraft may also be limited to flight ceilings of less than approximately 30,000 feet. The propulsion efficiency of a propulsion system may be defined as the thrust divided by the weight of the engine and propeller (or fan), often quoted as a ratio.
Basic turbofan engines typically have several times the thrust per unit weight (propulsion efficiency) as piston engines driving propellers, and so may be used to achieve aircraft speeds of 300-1200 mph where the thrust required to overcome aircraft drag may be higher than at low subsonic speeds, according to approximately the velocity squared. A turbofan may comprise a jet turbine and a propulsion fan, producing both reactive thrust and fan thrust. The fan itself, also sometimes called a rotor, may be constructed of a hub with a plurality of fan blades attached at its rim surface. A duct circumscribing the fan blades may mitigate tip turbulence and improve efficiency over propellers. Aircraft employing ducted turbofans may reach flight ceilings of approximately 50,000 to 60,000 feet. However, the multi-stage compressors and multi-stage turbines often contained within a basic turbofan may experience high heat and stresses, requiring superalloys or exotic metals, making turbofans expensive to build. Additionally, although basic turbofans may generate large amounts of thrust for a military aircraft, a turbofan may consume approximately three times as much fuel as a piston engine, which may make turbofan aircraft relatively expensive to fly. For example, a turbine may consume approximately one pound of fuel per horsepower per hour, whereas a piston engine may consume approximately ⅓rd pound per horsepower per hour. Additionally, the lower weight afforded by the turbofan's high propulsion efficiency may be partially cancelled out by the additional fuel that must be carried.
High bypass turbofans may derive most of their thrust from the ducted fan and little from the turbine exhaust, thereby reducing noise and making them ideal for commercial airliners and business aircraft operating at speeds of approximately 300-600 mph. Costs for a 4-6 passenger very light jet with a cruising speed of approximately 450 mph and a cruising altitude of approximately 40,000 feet may be at least approximately $3 million. Unfortunately, the long fan blades commonly used may have a low hub-to-tip ratio (HTR) producing a relative airflow that is subsonic at a root and supersonic at the tip of the blade. For example, a typical fan having a low HTR of 0.3 may, by reciprocal, create a differential in blade speed between the root and the tip, requiring a progressive blade twist to maintain a positive, non-stalling AOA. Additionally, a transonic zone occurring at the intersection of the subsonic and supersonic regions may generate shock wave turbulence that may require additional blade shaping to recover efficiency or may require additional power to overcome additional drag. The blade may be swept progressively backward to keep the leading edge behind a forward shock wave. Blade thickening may be necessary to stabilize a long blade against mechanical flutter, but may add weight and cause shock waves that reduce performance. In summary, thickening, sweeping, twisting, and otherwise shaping a blade in order to compensate for deficiencies in transonic, low HTR fans may require a complex manufacturing process not well suited to an economical high subsonic propulsion system.
A further drawback of a ducted fan and turbine combination optimized for propulsion efficiency is that the fan itself may not be optimized. To demonstrate this, fan efficiency may be defined as thrust divided by drive power, often quoted as pounds per horsepower. Due to the high propulsion efficiency of turbofans, the horsepower used to drive the fan is easily increased by making the turbine bigger and adding fuel which, when combined with a refinement in the fan, may result in more turbofan thrust, but less fan efficiency. For example, a longer blade may create more thrust, and additional drag that is overcome with a bigger turbine, resulting in a fan having less thrust per unit of drive horsepower but producing a turbofan with higher thrust to weight (propulsion efficiency). In conclusion, because the fans being used in high subsonic flight may be optimized in conjunction with a turbine, the fans themselves may not be efficient enough to propel a 2-4 seat aircraft at high subsonic speeds using a more economical reciprocating engine. This may be one reason why piston-driven ducted fans may not yet be efficient enough to reliably achieve high subsonic flight.
Another penalty of prior fan technology may be the use of multi-piece rotors that are heavier and may require that preformed fan blades be welded, bolted, or otherwise attached to a preformed hub, adding weigh and cost. For example, the complexly-shaped blades used in conventional fans may have a low HTR and may be therefore too heavy to be adhesively retained by a compositely formed hub. In addition, the materials of which the fan blade is fabricated may comprise exotic materials such as titanium and which may be too expensive or difficult to co-form with a hub. Also, conventional blades may be formed of heavy materials such as titanium, having a specific gravity of 4.5 grams per cubic centimeter (g/cc), or such as steel having a specific gravity of 7.8 g/cc, creating a higher centrifugal pull on a hub than a lighter material such as aluminum having a specific gravity of only 2.7 g/cc. Additionally, long blades are more susceptible to damage, such as by the ingestion of birds, necessitating an even thicker, heavier blade that precludes adhesion in a one-piece rotor. The result of a multi-piece assembly may be a higher parts count, complex manufacturing tooling, and a greater weight not supportive of economical high subsonic flight. What's needed is a rotor design that reduces the centrifugal pull of preformed fan blades on a composite hub having modest adhesive strength.
Another problem in fan art are the flow regimes that may arise from boundary layer conditions near the rim surface of the hub, and which may migrate to outboard regions of the blade, reducing lift and increasing drag. A flow regime may be a region of air having a localized pattern of movement distinct from air movement in adjacent regions. A flow regime may be a laminar flow over an airfoil, a vortex coming off of a wing tip, a boundary layer attached to a hard surface, a turbulent regime of air, such as on the suction side of a wing in stall, or a mix of these individual flow regimes. Boundary layers may be regions of shearing between the molecules of air attached to a hard surface and the air that is further away, giving rise to various flow regimes having turbulence, vortices, or other movement patterns. In contrast, the working portion of the fan blade may generate propulsion and an associated low pressure zone due to a laminar flow across the suction and pressure sides of the blade. Because inboard flow regimes near the rim surface may have differing airflow and higher pressures than the laminar flow generating propulsion, they may migrate outboard along the blade and substantially reduce propulsion.
Conventional turbofan designs may utilize various methods to compensate for inboard flow regimes in the fan, such as rounding the root of the blade so it does not attempt to generate lift, or using long blades to place the working portion of the blade further away from the rim surface, thereby forfeiting fan efficiency. However, while sacrificing efficiency may be acceptable in a design allowing higher fuel consumption and higher manufacturing cost, it may not be acceptable in a solution requiring economy. Particular inboard flow regimes that reduce fan efficiency may include those arising from the boundary layers associated with the rim surface, the air inlet adjacent to the rim surface, and the wing-body corner line between the hub and the root of the blade.
Another problem in the art may be the lack of a lightweight, composite rotor of simple manufacture. An integrally bladed composite rotor like that disclosed in U.S. Pat. No. 7,491,032 may form blades at each blade location during circumferential winding of a hub with a continuous filament, creating a lightweight one piece assembly. Unfortunately, the disclosure requires cornering of the filaments from a circumferential to a radial path to form each blade, then back again to a circumferential path, which may require a complex manufacture and tension control. Also, blade shaping options may be fewer in such an integrated rotor formation since any sweep, twist, thickness, and taper that is required needs to be integrated into one winding process, which may restrict the features and parameter ranges can be implemented. Additionally, the disclosure of an integrally bladed rotor may not allow for the insertion of a simple preformed blade into a hub being wound.
Another example of a composite rotor is the composite turbine described in U.S. Pat. No. 4,354,804. The hub disclosed in '804 may be formed of carbon cloth and reinforcing carbon filaments, and the blade may be formed of chopped carbon fibers with radial reinforcing, all fabricated at the same time. Unfortunately, complex shaping of blade and hub are combined into one process that may be expensive and a difficult one in which to control tolerances. In another disclosure, a composite flywheel disclosed by U.S. Pat. No. 4,187,738 uses continuous filaments coated with a binding agent to layer concentric toroids, each layer being individually cured before adding the next layer. However, layering and then curing successive toroids of composite material may require a long manufacturing process. Additionally, the process disclosed in '738 may require that the filaments be highly prestressed in order to resist the large centrifugal forces present in a flywheel rotating at speeds in excess of 35,000 rpm. Unfortunately, prestressing may be an expensive and unnecessary manufacturing constraint for a lower-speed fan hub. For example, a fan comprised of low-stress filament may be adequately strong for speeds of less than approximately 14,000 revolutions per minute (rpm). Additionally, rotor speeds of less than 14,000 rpm may allow grease bearings to be used instead of complicated oil lubrication. In summary, the prior art may lack a method for manufacturing a lightweight composite rotor of modest centrifugal strength using preformed blades and using simple manufacturing techniques.
As can be seen, there exists a need in the art for a more efficient and lightweight ducted fan, preferably driven by a reciprocating engine, and optimized for high subsonic flight. Furthermore, there exists a need in the art for an all-supersonic rotor that eliminates transonic turbulence and simplifies blade shaping. Additionally, there exists a need in the art for methods to manage inboard flow regimes that reduce fan efficiency. Also, there exists a need in the art for a composite hub comprised of low-stress filaments into which thin, preformed blades may be adhesively retained, forming a one-piece rotor. Finally, there exists a need in the art for a composite rotor formed of non-exotic materials and that can be fabricated using simple processes and without expensive machining or tooling.